The state of the art is well summarized in a report prepared by Henry Persson and entitled "Design, Equipment and Choice of Tactics are Critical When Fighting Large Tank and Bund Fires." That report is further identified as Brandforsk Project: 612-902; Swedish National Testing and Research Institute, Fire Engineering, SP Report 1992:02. The purpose of that report was to develop knowhow based on the experience and recommendations of fire experts to provide a practical basis for the design and planning of foam extinguishing systems for large tank and bund fires. A bund fire is one within the embankment or dike surrounding a storage tank.
A traditional approach to the fighting of such fires has been to direct streams of water and foam onto the fire site through monitors or even hand-held nozzles. In order to successfully extinguish large fires using traditional techniques it is necessary to have available an adequate supply of water and foam concentrate to allow the application of foam liquid at a minimum rate of 6.5 l/m.sup.2 /min to the burning surface for some 60 to 90 minutes. The report indicates agreement among the experts that concentrating foam application on as small an area in the tank as possible is far superior to the previously accepted technique of fighting tank fires with several small monitors distributed around the circumference of the tank. Concentrating the foam application at one point more quickly establishes a bridge head, or initial foam cover, thus increasing the effectiveness of subsequently applied foam.
Among other findings of the report are that no successful fire extinguishing operation has been verified in tanks over about 45 m in diameter. Indeed, some experts hold that a tank of 45 m in diameter represents about the largest that can be extinguished with mobile equipment. It appears to be the general consensus of the experts that tanks up to at least 60 m in diameter can be extinguished if the tanks are equipped with fixed "over-top" systems to apply foam. It is considered possible as well to extinguish fires in even larger tanks if the over-top system is supplemented with a bottom feed system.
A fixed, over-top system comprises permanently installed piping and foam sprinkler nozzles within the tank itself at a level above the liquid surface when the tank is filled to capacity. A bottom feed system employs a hose array with foam deploying nozzles adapted to float on the surface of the stored liquid and to rise and fall with the liquid as the tank is filled and emptied. Both systems require connection to a water source and to a supply of foam concentrate. That connection may be a permanent one through a direct attachment to the water mains and to a store of foam but the systems are more commonly supplied from a mobile unit which is connected to a system through hoses at the time of need. Both systems are difficult to maintain and are essentially impossible to test without contamination of the tank contents.
Subsurface foam injection systems for fighting tank fires are also known. Those systems inject foam through a port in a tank wall under sufficient pressure to overcome the pressure of the head of gasoline or other flammable liquid stored in the tank. Foam then rises through the liquid in the tank and spreads on the liquid surface to extinguish the fire. Production of the foam is accomplished using a high back pressure foam-maker. Such foam-makers operate by using a venturi to aspirate air into a foam solution at a carefully selected ratio of air-to-solution ranging from 2:1 to 4:1. High back pressure foam-makers usually operate at inlet pressures of 100 to 300 psi against back pressures (the sum of the tank fluid head pressure and the piping friction losses between the foam-maker and the tank) of up to about 40% of the inlet pressure.
Application rate, or injection rate, of the foam solution is governed by the tank surface area. That application rate is typically set at about 4 lpm/m.sup.2 and seldom exceeds twice that rate. Thus, for a tank 50 m in diameter, the foam solution injection rate would be some 7850 lpm. The quantity of foam solution required is dependent upon the vapor pressure, or flash point, of the flammable liquid stored in the tank. A liquid with a high flash point, lubricating oil for example, would typically require foam injection for about a half hour while a volatile fuel such as gasoline requires a foam injection time of about twice that. Thus, the volume of foam solution required to extinguish a fire in a gasoline tank amounts to nearly a foot of tank depth.
Foam concentrates used with subsurface injection systems must be either a fluoroprotein or an AFFF type. Other foam types absorb hydrocarbon vapor as the foam rises through a fuel column and will burn as the foam reaches the surface. Conventional subsurface foam injection systems can be used only with hydrocarbons and not with polar liquids. Polar liquids tend to dissolve fluoroprotein or AFFF foam concentrates causing water to drain from the foam bubbles and leaving only the aspirated air to rise to the liquid surface. For that same reason, foam produced using a high back pressure foam maker cannot be discharged into the layer of water typically found in the bottom of hydrocarbon storage tanks.
Fires aboard tankers carrying either crude oil or refined petroleum products pose many of the same problems as do fires in stationary tanks. Consequently, tanker fires have been traditionally fought using much the same tactics used in the fighting of stationary tank fires. However, the difficulties of access and of the coordination of equipment, personnel and decision-making are ordinarily vastly greater in a tanker fire than those encountered at land locations.
Fires in tankers and stationary storage tanks, while relatively uncommon, pose enormous risks. Those risks include the threat of injury or death to people aboard the ship or in the area or engaged in fighting the fire, the likelihood of huge property losses, and the nearly certain contamination of soils, beaches, ground and surface water and air. Further, the intense thermal radiation always threatens to ignite adjacent structures and tanks thus compounding the risks and increasing the potential losses.
With this background, it can readily be appreciated that fire fighting tactics and systems which can more quickly and surely bring under control and extinguish fires in confined spaces in structures, aboard tankers and in tanks, particularly those fires in large stationary or mobile tanks, is of great environmental and economic importance.